In a powder metallurgy process, source powders are mixed together; the mixture is transferred, is filled into a die, and is then pressed into a formed body (hereinafter referred to as a green compact); and the green compact is withdrawn from the die and is then subjected to a post-treatment such as sintering as required.
In the powder metallurgy process, in order to achieve an increase in product quality and a reduction in production cost, it is necessary to ensure all of high powder flowability in a transferring step, high compressibility in a pressing step, and low ejection force in a step of withdrawing the green compact from the die.
As for techniques for improving the flowability of iron-based mixed powders, PTL 1 discloses that the flowability of an iron-based mixed powder can be improved by adding a fullerene thereto.
PTL 2 discloses a technique for improving the flowability of powder by adding a particulate inorganic oxide with an average particle size of less than 500 nm thereto.
However, the use of these techniques is insufficient to ensure high compressibility and low ejection force while flowability is maintained.
In order to increase the density of a green compact or in order to reduce the ejection force thereof, it is effective to use a lubricant that has ductility and that is soft at a temperature at which an iron-based mixed powder is pressed. This is because the lubricant seeps out of the iron-based mixed powder during pressing to adhere to a surface of a die and therefore reduces the friction between the die and the green compact.
However, the lubricant has ductility and therefore is likely to adhere to particles of an iron powder and powder for an alloy. Hence, there is a problem in that the flowability and filling ability of iron-based mixed powder are impaired.
The blending of the above carbon material, fine particles, and lubricant reduces the theoretical density (supposing that the voidage is zero) of the iron-based mixed powder to cause a reduction in green density; hence, it is not preferable to blend large amounts of these materials.
It has been extremely difficult to balance the flowability of a conventional iron-based mixed powder, high green density, and low ejection force.